Programmable/tunable active RC filter

ABSTRACT

A programmable/tunable active low-pass filter at least has the resistors, capacitors and shunt control means. It uses the resistor ladder that is structured with various fixed resistors to implement the shunt control means. The cut-off frequency of a filter is associated with the time constant, which is determined by equivalent resistance and equivalent capacitance value in the filter circuit; therefore, the filter of the present invention allowed users to fine tune the cut-off frequency linearly through the shunt control means when the variation of the environment or process parameters of manufacture cause the cut-off frequency drift, thus, the cut-off frequency can be kept in a constant value. The present invention also provides a means for programming the cut-off frequency to a desirous frequency value dependent upon the conditions of application within a big range. Thus, it can be used in many purposes widely with the same filter circuit.

BACKGROUND OF THE PRESENT INVENTION

1. Field of the Invention

The invention generally relates to analog filter circuits, and moreparticularly to a method and apparatus for fine tuning and adjusting thecut-off frequency and voltage gain of an active filter.

2. Description of the Prior Art

A filter is an important and familiar component in signal processingsystems; and the purpose of the filter is to eliminate the band signalsunwanted for retaining and amplifying the band signals desired.Integrating the filters into one chip is a common trend in themanufacture process of the advanced integrated circuit, especially tothe communication markets. As the communication system continuouslydevelops, the signal-processing circuits not only require highperformances, but also should have a multi-function for a system toprocess different types of signals. For instance, the 2nd/3rd generationcell phone in wireless communication and V/ADSL (i.e. very highdata/asymmetrical digital subscriber line) modem in wired communication,they all require the filters that can be switched in the both systems.Therefore, how to design a filter, which can program the cut-offfrequency to an accurate value within a large range, is a main key pointin today's signal processes of the communication systems.

The cut-off frequency of a filter is in direct proportion to thereciprocal of time constant τ (i.e. f=½πτ), and the time constant τ isthe product of equivalent resistance and capacitance (i.e. τ=R×C) of theactive RC filter. In general, the programmable active RC filters finelytune the value of the cut-off frequency by adjusting the number ofoperative cells of the resistance or the capacitance. FIG. 1A depicts aconventional programmable filter with a selective parallel-capacitorcircuit structure, the situations (off and on) of the switch devices b0,b1, b2, b3 can determine whether the capacitors connected with thefilter are operative or not. Since each of the capacitors is connectedin parallel, so that the equivalent capacitance of the filter circuitcan be obtained by adding the capacitance of the capacitors connectedwith the switch devices b0, b1, b2, b3 that is operative. For instance,the equivalent capacitance of the circuit is 0 when the switch devicesb0, b1, b2, b3 are all turned off; similarly, the equivalent capacitanceof the circuit is 15C when the switch devices b0, b1, b2, b3 are allturned on. Thus, the combination of the switch devices b0, b1, b2, b3 isthe control code of the circuit that can be used to control theequivalent capacitance of the filter and then can achieve the purpose offine tuning the cut-off frequency. In addition, it can also obtain thesame purpose by using the selective parallel-resistor circuit (referringto FIG. 1B) or the selective series-resistor circuit (referring to FIG.1C) structure to substitute for resistor R1 in FIG. 1A.

According to the fundamental principle of active filters, the cut-offfrequency is in direct proportion to the reciprocal of the product ofequivalent resistance and equivalent capacitance of the circuit (i.e.f=½πτ=1/(2πR×C)). Hence, the relation between the control code and thecut-off frequency of the filter is non-linear regardless of equivalentresistance or equivalent capacitance is tuned, which is shown in FIG.2A. In applications, besides, when the cut-off frequency is requiredthat being adjusted within certain accuracy, the number of control bitswill be determined by the largest slope parts in FIG. 2A. It istherefore that, it will increase the number of devices of the circuitsand reduce the control efficiency; thus, it will make it becomedifficult to design in an integrated circuit. In addition, since thecut-off frequency of such kind of filters can only be tuned within afixed range, the applications of that are limited in the kind offilters.

Accordingly, it can become a linear relation (as is shown in FIG. 2B)between the control code and the cut-off frequency by making therelations between the control code and the reciprocal of equivalentresistance (i.e. the equivalent conductance) of the filter become morelinear. Therefore, it can not only increase the control efficiency andreduce the area of the circuits, but also extend the tuning range ofcut-off frequency and add the applications of the circuits for themulti-function purpose.

SUMMARY OF THE PRESENT INVENTION

As is described above, the problems of techniques in the prior art arelimited in applications, and with low control efficiency and largecircuit area; thus, one of the purposes of the present invention is toprovide a filter circuit having an utility of tuning the cut-offfrequency; in this regard, it can make the cut-off frequency be shiftedwithin a tuning rage and be tuned to a desired band according to thepractical applications for achieving the multi-function purpose.

Another one of the purposes of the present invention is to provide afilter circuit having an utility of tuning the cut-off frequency thatcan narrow the tuning range of the cut-off frequency and increase thelevel of tuning accuracy with a constant number of control bits forachieving a higher control resolution.

Still another one of the purposes of the present invention is to providea fine tune apparatus having a linear relation with the cut-offfrequency of a filter circuit; in this regard, it can precisely tune thecut-off frequency to a desired value for compensating the parametervariations due to the manufacture process or the environments. Besides,it has a smaller circuit area under the same accuracy.

Still another one of the purposes of the present invention is to providea voltage gain tuning apparatus of a filter for tuning the voltage gainafter the tuning of the cut-off frequency are tuned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a circuit diagram of the active filter with selectiveparallel-capacitor construct;

FIG. 1B is a circuit diagram of the selective parallel-resistorconstruct;

FIG. 1C is a circuit diagram of the selective series-resistor construct;

FIG. 2A is a relation diagram between the cut-off frequency and controlcode of the general active filters;

FIG. 2B is a relation diagram between the cut-off frequency and controlcode of the present invention;

FIG. 3 is a circuit diagram of an improved active filter;

FIG. 4 is a circuit diagram of the four degrees resistor ladderconstruct;

FIG. 5 is a circuit diagram of an embodiment according to the presentinvention; and

FIG. 6 is a circuit diagram of an embodiment according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following descriptions about the circuit of present invention notinclude the complete structure of the active filter. It just quotes thekey points of traditional techniques for illustrates the presentinvention. Moreover, all of the drawings relates to the presentinvention don't accord the scale, they are just used to represent thecharacteristics of structure of present invention.

FIG. 3 depicts an improved active filter circuit according to thepresent invention. The circuit is composed of a differential amplifierD_(Amp) with two input terminals and two output terminals, which arerespectively connected to the four controllable shunt devices 302, 304,306 and 308 (for instance, resistor ladders, RL); and two identicalcapacitance component C_(f), wherein each of the switch devicesrespectively used in controllable shunt devices 302, 304, 306 and 308are exactly the same. The voltage signal inputted from the inputterminals of the filter can be transformed into a current signal bymeans of the equivalent resistance of the controllable shunt devices 302and 304. The controllable shunt devices can be controlled by thesituations (on and off) of each switch device for achieving the purposeof operating under shunt control. Furthermore, the switch devices can becontrolled synchronously by the system attached (for instance, digitalmobile phone system); otherwise, they can be controlled individually(i.e. asynchronously).

FIG. 4 depicts a four-degree R-2R resistor ladder, which is composed oftwo resistors (R1, R2) and four switch devices (b3, b2, b2, b0), whereinthe resistance of R2 is twice as big as the resistance of R1. In theembodiment, the controllable shunt devices (302, 304, 306, 308) of theactive filter circuit according to the present invention are implementedby four R-2R resistor ladders with a single input terminal and twooutput terminals. Further, it utilizes the same set of control code tocontrol the on and off situations of the switch devices (b3, b2, b1, b0)in the controllable shunt device (302, 304, 306, 308) synchronously.Besides, all switch devices of the embodiment are implemented by thethree-terminal devices with the same property; for instance, the fieldeffect transistors (FETs) with the same physical size. Since the fieldeffect transistors and the controllable shunt devices are manufacturedby the same manufacturing process, and the operating environments ofthem are coincident; thus, the mismatch effect between the equivalentresistances caused by the body effects will not appear.

Next, referring now to FIG. 3, each of the input terminals P_(i1),P_(i2) of the controllable shunt devices 302, 304 is respectivelyconnected to the input terminals V_(ip), V_(in) of the circuit; each ofthe output terminals P_(o1), P_(o3) of the controllable shunt devices302, 304 is respectively connected to the positive and the negativeinput terminals of differential amplifier D_(Amp); and each of anotheroutput terminals P_(o2), P_(o4) of the controllable shunt devices 302,304 connects to each other for generating a loop. Further, the inputterminal P_(i3) of the controllable shunt device 306 is connected to theoutput terminal V_(on) of the circuit and the negative output terminalof the differential amplifier D_(Amp); while the input terminal P_(i4)of the controllable shunt device 308 is connected to the output terminalV_(op) of the circuit and the positive output terminal of thedifferential amplifier D_(Amp). Each of the output terminals P_(o5),P_(o7) of the controllable shunt devices 306, 308 feeds back to thepositive and negative input terminals of the differential amplifierD_(Amp); and similarly, each of another output terminals P_(o6), P_(o8)of the controllable shunt devices 306, 308 connects to each other forgenerating a loop. Besides, each of the two capacitors C_(f) isrespectively connected to the input terminals and the output terminalsof the differential amplifier D_(Amp). Accordingly, it can linearlycontrol the current magnitude of the equivalent signals by changing theswitches of the resistor ladders, and it can further linearly controlthe cut-off frequency of the active filter. In this regard, it can beseen in the following analysis.

By using the equivalent half-circuit analysis method to analyze thecircuit in FIG. 3 and draw the signal flow graph (i.e. SFG); thus, thefollowing transfer function can be obtained:$\frac{Vo}{Vi} = \frac{- \frac{a}{{RL} \cdot {Cf}}}{S + \frac{a}{{RL} \cdot {Cf}}}$wherein V_(o) is the differential output signal (i.e.V_(o)=V_(op)−V_(on)); V_(i) is the differential input signal (i.e.V_(i)=V_(ip)−V_(in)); S is a frequency domain variable after the Fouriertransform of the time domain; parameter “a” is the ratio of the currentflowing out of the output terminal P_(o1) of the RL circuit in FIG. 4 tocurrent I_(r); thus, the variable range of the parameter a is 0, 1/16,2/16, . . . , 14/16, 15/16. Regarding the resistor R2 that the currentI_(r)/16 passed through, if the terminal connected to the outputterminal P_(o2) is changed to being connected to the output terminalP_(o1), then the variable range of the parameter a is 1/16, 2/16, . . ., 15/16, 16/16. According to the above-mentioned equation, the cut-offfrequency ω of the filter is:$\omega = {\frac{a}{{RL} \cdot {Cf}}\left( {{rad}\text{/}\quad s} \right)}$

Since the parameter a has a linear variation with the switch devices b3,b2, b1, b0; thus, there has a linear relation between the cut-offfrequency ω and switch devices b3, b2, b1, b0 of the filter. Forinstance: when  a = 0, ω = 0  (rad/s)${{{when}\quad a} = 1},{\omega = {\frac{1}{{Cf} \cdot {RL}}\left( {{rad}\text{/}s} \right)}}$

FIG. 5 depicts another preferred embodiment of the present invention.First, four variable resistors R_(s) are used to respectively connectthe input terminals P_(i1), P_(i2), P_(i3), P_(i4) and the outputterminals P_(o1), P_(o3), P_(o5), P_(o7) of the controllable shuntdevices 302, 304, 306, 308, and a variable resistor R_(c) is used tocascade the above-mentioned terminals; wherein the parallel variableresistors R_(s) and the series variable resistors R_(c) can beselectively implemented by the resistor ladders, the selectiveparallel-resistor structure or the selective series-resistor structure.Further, the selective series-resistor structure (as shown in FIG. 1C)is chosen for variable resistor R_(s) and R_(c) in this embodiment.Besides, in FIG. 5, R_(c) is the same circuit structure as R_(c1) andR_(c2) (i.e. R_(c)=R_(c1)=R_(c2)); thus, R_(c) is used for representingR_(c1) and R_(c2) when under transferring of a transfer function.

By using the equivalent half-circuit analysis method to analyze thecircuit in FIG. 5 and draw the signal flow graph (i.e. SFG); thus, thefollowing transfer function can be obtained:$\frac{Vo}{Vi} = \frac{\frac{- \left( {{a\frac{Rs}{{Rs} + {RL}}} + \frac{RL}{{Rs} + {RL}}} \right)}{C\quad\left( {{Rc} + \frac{{Rs} \cdot {RL}}{{Rs} + {RL}}} \right)}}{S + \frac{\left( {{a\frac{Rs}{{Rs} + {RL}}} + \frac{RL}{{Rs} + {RL}}} \right)}{C\quad\left( {{Rc} + \frac{{Rs} \cdot {RL}}{{Rs} + {RL}}} \right)}}$wherein the cut-off frequency ω of the filter is:$\omega = {\frac{{a\frac{Rs}{{Rs} + {RL}}} + \frac{RL}{{Rs} + {RL}}}{C\quad\left( {{Rc} + \frac{{Rs} \cdot {RL}}{{Rs} + {RL}}} \right)}\left( {{rad}\text{/}s} \right)}$and${{{when}\quad a} = 0},{\omega = {\frac{1}{C\quad\left( {{Rs} + {{Rc}\frac{{RL} + {Rs}}{RL}}} \right)}\left( {{rad}\text{/}s} \right)}}$${{{when}\quad a} = 1},{\omega = {\frac{1}{C\quad\left( {{Rc} + \frac{{Rs} \cdot {RL}}{{Rs} + {RL}}} \right)}\left( {{rad}\text{/}s} \right)}}$

According to the above-described equation, the cut-off frequency stillhas a linear variation with the switch devices b3, b2, b1, b0. It canalso be appreciated that the slope of the linear variation of thecut-off frequency relative to the parameter “a” is direct proportion toR_(s)/(R_(s)+RL); it is therefore that the tuning range of the cut-offfrequency can be controlled by an appropriate design of the ratiobetween the variable resistor R_(c) and the equivalent resistance of theresistor ladder RL. Next, the cut-off frequency can be lowered byincreasing the resistance of the variable resistor R_(c); adjust thecut-off frequency of the filter to a frequency band by tuning theresistance of the variable resistor R_(c) and it is therefore that thefilter can be suitable for the multi-operation modes system structure.In addition, it is an inverse proportion between the variable resistorR_(c) and the equivalent resistance of the resistor ladder RL when thecut-off frequency is the same; that is, the equivalent resistance of theresistor ladder RL can be lowered (i.e. the resistances of R1 and R2 ofthe resistor ladder RL can be reduced) when the resistance of thevariable resistor R_(c) is increased; accordingly, the area of thecircuit will be greatly reduced. Besides, when tuning the resistanceduring the manufacture process of the integrated circuit, the variableresistors R_(c) and R_(s) can be trimmed by means of a trimming method(for instance, laser trimming) for achieving the purpose of fine tuning;and there is no need to synchronously trim all resistors of R-2Rresistor ladders.

As the above-mentioned descriptions, when the filter circuit in FIG. 5is under operation, the cut-off frequency of the filter can bepositioned to a desired band by tuning the series variable resistors atfirst; further, it is determined as regards the application fields. Forinstance, when a filter is applied to a component of a GSM/CDMA (i.e.global standard for mobile/code division multiple access) dual modulesystem, the cut-off frequency can be positioned in a frequency band thatis determined by series variable resistors R_(c). The frequency band isadjusted to a frequency band or selected in a wireless communication;for instance, to a GSM/CDMA dual module system, the frequency band isGSM band or CDMA band. Further, to narrow the tuning range of thecut-off frequency by tuning the parallel variable resistors R_(s), andthen the cut-off frequency can approach to the desired value and thecontrol resolution can be increased too. Moreover, control the switchdevices (b3, b2, b1, b0) of the controllable shunt devices (306, 308) byusing the control code for fine tuning the cut-frequency to a desiredvalue. And then, the signal outputted from the filter can be sent to thetransceiver of the dual module system. Finally, when the cut-offfrequency has been tuned to a desired value, adjust the control switchdevices (b3, b2, b1, b0) of the controllable shunt devices (302, 304)for programming the voltage gain of the active RC filter.

In addition, in the embodiment of FIG. 5, there is an alternative methodthat tuning the parallel variable resistors R_(s) and then tuning theseries resistors R_(c). In the embodiment of the present invention, themethod that tuning the series resistors R_(c) and then tuning theparallel variable resistors R_(s) is chosen. Further, about tuning thecut-off frequency, there is an alternative method that tuning theselective parallel-capacitor circuit (as shown in FIG. 1A). However,since the selective parallel-capacitor circuit would cause nonlineareffects, so that the embodiment adopts the method that tuning theresistors for achieving the purpose of fine tuning.

Besides, since assuming that the controllable shunt devices (302, 304,306 and 308) are fully the same R-2R resistor ladders and each of themis controlled by means of a synchronous control method; according to theprinciple of the active filters, it is realized that the voltage gain ofthe circuit is a fixed value. Accordingly, when the cut-off frequency isfinely tuned to a desired value and then being fixed, thus controllingof the controllable shunt devices 306, 308 can be stopped. Next, tosynchronously control the control switch devices (b3, b2, b1, b0) of thecontrollable shunt devices (302, 304) for further changing theequivalent resistance of the controllable shunt device 302 or 304;therefore, it can adjust the voltage gain of the active RC filter forthe purpose of adjustable, programmable and changeable tuning of thevoltage gain. Further, according to the circuit structure, it isrealized that when the filter circuit has finished frequency tuning, itcan also achieve the purpose of adjustable, programmable and changeabletuning of the voltage gain by only tuning the resistor R_(c1).

FIG. 6 depicts another preferred embodiment of the present invention. Itis an altered circuit of the circuit in FIG. 5 for simplifying thecontrol procedure; wherein the number of the controllable shunt devicesis simplified from four to two (i.e. 602 and 604 in FIG. 6). Further,the structure of that is the same as the R-2R resistor ladders of FIG.4, and each of the two variable resistors R_(p) is respectivelyconnected to the two input terminals V_(ip), V_(in) and then each of thetwo variable resistors R_(f) feeds back between R_(p) and R_(c) throughthe two output terminals V_(op), V_(on). Similarly, the variableresistors R_(f) and R_(p) can be implemented by the resistor ladders,the selective parallel-resistor structure (such as the circuit shown inFIG. 1B) or the selective series-resistor structure (such as the circuitshown in FIG. 1C). In the embodiment, the variable resistors R_(f) andR_(p) are implemented by selective series-resistor circuits for reducingthe numbers and the area of resistor ladders.

By using the equivalent half-circuit analysis method to analyze thecircuit in FIG. 6 and draw the signal flow graph (i.e. SFG); thus, thefollowing transfer function can be obtained:$\frac{Vo}{Vi} = \frac{- {\frac{Rf}{Rp} \circ \frac{\frac{Rp}{{Rp} + {Rf}}\left( {{a\frac{Rs}{{Rs} + {RL}}} + \frac{RL}{{Rs} + {RL}}} \right)}{C\quad\left( {{Rc} + \frac{{Rs} \cdot {RL}}{{Rs} + {RL}} + \frac{{Rp} \cdot {Rf}}{{Rs} + {RL}}} \right)}}}{S + \frac{\frac{Rp}{{Rp} + {Rf}}\left( {{a\frac{Rs}{{Rs} + {RL}}} + \frac{RL}{{Rs} + {RL}}} \right)}{C\quad\left( {{Rc} + \frac{{Rs} \cdot {RL}}{{Rs} + {RL}} + \frac{{Rp} \cdot {Rf}}{{Rs} + {RL}}} \right)}}$wherein the cut-off frequency ω of filter is:$\omega = {\frac{\frac{Rp}{{Rp} + {Rf}}\left( {{a\frac{Rs}{{Rs} + {RL}}} + \frac{RL}{{Rs} + {RL}}} \right)}{C\quad\left( {{Rc} + \frac{{Rs} \cdot {RL}}{{Rs} + {RL}} + \frac{{Rp} \cdot {Rf}}{{Rp} + {Rf}}} \right)}\left( {{rad}\text{/}s} \right)}$and${{{when}\quad a} = 0},{\omega = {\frac{1}{C\quad\left\lbrack {{{Rc}\quad{\left( {1 + \frac{Rf}{Rp}} \right) \cdot \left( {1 + \frac{Rs}{RL}} \right)}} + {{Rs}\quad\left( {1 + \frac{Rf}{Rp}} \right)} + {{Rf}\quad\left( {1 + \frac{Rs}{RL}} \right)}} \right\rbrack}\left( {{rad}\text{/}s} \right)}}$${{{when}\quad a} = 1},{\omega = {\frac{1}{C\quad\left\lbrack {{\left( {{Rc} + \frac{{Rs} \cdot {RL}}{{Rs} + {RL}} + {Rc}} \right) \cdot \frac{{Rp} + {Rf}}{Rp}} + {Rf}} \right\rbrack}\left( {{rad}{\text{/}\text{s}}} \right)}}$

According to the above-described equation, it can be appreciated thatthe circuit has the same properties with the circuit in FIG. 5. Further,there are only two controllable shunt devices used to tune the cut-offfrequency of the filter; thus, it can greatly reduce the area andcomplexity of the circuit. And next, the direct voltage gain of thefilter can be tuned by the resistances of the variable resistors R_(f)and R_(p); thus, The direct voltage gain of the filter can be tuned bythe resistances of the two variable resistors. In the embodiment of thepresent invention, R_(p) is chosen for tuning the direct voltage gain ofthe filter.

Besides, when the cut-off frequency is fixed, higher direct voltage gainof the filter makes smaller resistance R_(p) that is needed; therefore,the area of the circuit can be reduced greatly. Similarly, the tuningrange of the cut-off frequency can be controlled by an appropriatedesign of the ratio between the variable resistor R_(c) and theequivalent resistance of the resistor ladder RL. Then, adjust thecut-off frequency of the filter to a frequency band by tuning theresistance of the variable resistor R_(c) and it is therefore that thefilter can be suitable for the multi-operation modes system structure.

Finally, since the noise frequency spectrum density of the filtercircuit is determined by the equivalent resistance; thus, when theoperation band of the filter is needed to change in a wide range, tuningof the equivalent capacitance is available to avoid the substantialvariation of the noise frequency spectrum density caused by tuning ofthe equivalent resistance. Accordingly, the capacitance components inthe circuits of FIG. 5 and FIG. 6 are implemented by a variablecapacitor “C”. All of the variable capacitors and resistors in theembodiments of the present invention can be implement by any prior arts;for instance, the selective parallel-capacitance structure shown in FIG.1A, the selective parallel-resistor structure shown in FIG. 1B and theselective series-resistor structure shown in FIG. 1C.

The types of the variable resistor apparatus in the present inventionare not restricted except the controllable shunt devices (should beimplemented by the resistor ladder circuits), and it can be implementedby the resistor ladders, selective parallel-resistor and selectiveseries-resistor circuits according to the criterion of the designer. Theembodiment in FIG. 5 can be adopted when the filter circuit needs aprecise cut-off frequency, and the embodiment in FIG. 6 can be adoptedwhen the filter circuit needs a simpler control procedure and a smallersize.

While this invention has been described with reference to illustrativeembodiments, this description does not intend or construe in a limitingsense. Various modifications and combinations of the illustrativeembodiments, as well as other embodiments of the invention, will beapparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

1. A programmable/tunable active RC filter, comprising: a firstcontrollable shunt device having an input terminal, a first outputterminal and a second output terminal, wherein said input terminal ofsaid first controllable shunt device is connected to a first inputsignal; a second controllable shunt device having an input terminal, afirst output terminal and a second output terminal, wherein said inputterminal of said second controllable shunt device is connected to asecond input signal, and said second output terminal of said secondcontrollable shunt device is connected to said second output terminal ofsaid first controllable shunt device; a third controllable shunt devicehaving an input terminal, a first output terminal and a second outputterminal; a fourth controllable shunt device having an input terminal, afirst output terminal and a second output terminal, wherein said secondoutput terminal of said fourth controllable shunt device is connected tosaid second output terminal of said third controllable shunt device,said first, second, third and fourth controllable shunt devices fordetermining a cut-off frequency; a differential amplifier having twoinput terminals and two output terminals, wherein each of said two inputterminals is respectively connected to each of said first outputterminals of said first controllable shunt device, said secondcontrollable shunt device, said third controllable shunt device and saidfourth controllable shunt device, each of said two output terminals isrespectively connected to each of said input terminals of said thirdcontrollable shunt device and said fourth controllable shunt device; andtwo capacitors, wherein each of said two capacitors is respectivelyconnected to said two input terminals and said two output terminals ofsaid differential amplifier for forming a RC filter and then beingoutputted from two output terminals of said two capacitors.
 2. Theprogrammable/tunable active RC filter according to claim 1, furthercomprising two first variable resistors R_(c1), each one end of said twofirst variable resistors is respectively connected to said first inputsignal and said second input signal, and each another end of said twofirst variable resistors is respectively connected to each of said inputterminals of said first controllable shunt device and said secondcontrollable shunt device.
 3. The programmable/tunable active RC filteraccording to claim 1, further comprising two second variable resistorsR_(c2), each one end of said two second variable resistors isrespectively connected to said two output terminals of said differentialamplifier, and each another end of said two second variable resistors isrespectively connected to each of said input terminals of said thirdcontrollable shunt device and said fourth controllable shunt device. 4.The programmable/tunable active RC filter according to claim 1, furthercomprising four tunable feedback resistors R_(S), each of said fourtunable feedback resistors is respectively connected to said inputterminals and said first output terminals of said first controllableshunt device, said second controllable shunt device, said thirdcontrollable shunt device and said fourth controllable shunt device.5-12. (canceled)
 13. The programmable/tunable active RC filter accordingto claim 1, wherein said first controllable shunt device, said secondcontrollable shunt device, said third controllable shunt device and saidfourth controllable shunt device determines said cut-off frequency bymeans of a synchronous control method.
 14. The programmable/tunableactive RC filter according to claim 1, wherein each of said firstcontrollable shunt device, said second controllable shunt device, saidthird controllable shunt device and said fourth controllable shuntdevice respectively determines said cut-off frequency by means of anindividual control method.
 15. The programmable/tunable active RC filteraccording to claim 1, wherein said first controllable shunt device isadjusted by means of a programmable method for adjusting saidprogrammable/tunable active RC filter to a voltage gain.
 16. Theprogrammable/tunable active RC filter according to claim 3, wherein saidtwo first variable resistors and said two second variable resistors areadjusted by means of a synchronous control method for adjusting saidprogrammable/tunable active RC filter to a frequency band.
 17. Theprogrammable/tunable active RC filter according to claim 3, wherein saidtwo first variable resistors and said two second variable resistors aretrimmed by means of a trimming method for adjusting saidprogrammable/tunable active RC filter to a frequency band.
 18. Theprogrammable/tunable active RC filter according to claim 4, wherein saidfour tunable feedback resistors are adjusted by means of a synchronouscontrol method for adjusting said programmable/tunable active RC filterto a frequency tuning range.
 19. The programmable/tunable active RCfilter according to claim 4, wherein said four tunable feedbackresistors are trimmed by means of a trimming method for adjusting saidprogrammable/tunable active RC filter to a frequency tuning range. 20.The programmable/tunable active RC filter according to claim 2, whereinsaid first variable resistor is adjusted by means of a programmablemethod for adjusting said programmable/tunable active RC filter to avoltage gain.
 21. A method of carrying out a programmable/tunable activeRC filter, comprising: adjusting said programmable/tunable active RCfilter to an operation band by tuning a plurality of series variableresistors; determining a cut-off frequency of said programmable/tunableactive RC filter within the tuning frequency range by tuning a pluralityof controllable shunt devices; and programming said programmable/tunableactive RC filter to a voltage gain after said cut-off frequency has beendetermined by tuning parts of said plurality of controllable shuntdevices.
 22. The method according to claim 21, further comprising:adjusting said programmable/tunable active RC filter to a frequencytuning range by tuning a plurality of parallel variable resistors. 23.The method according to claim 21, wherein said plurality of controllableshunt devices are resistor ladder circuit structures.
 24. The methodaccording to claim 21, wherein each of said plurality of series variableresistors is composed of a plurality of parallel-resistors.
 25. Themethod according to claim 22, wherein each of said plurality of parallelvariable resistors is composed of a plurality of parallel-resistors. 26.The method according to claim 21, wherein each of said plurality ofseries variable resistors is composed of a plurality ofseries-resistors.
 27. The method according to claim 22, wherein each ofsaid plurality of parallel variable resistors is composed of a pluralityof series-resistors.
 28. The method according to claim 23, wherein saidresistor ladder structures control a plurality of switch devices bydigital control codes for determining an equivalent resistance.
 29. Themethod according to claim 21, further comprising tuning a variablecapacitor before adjusting the operation band.
 30. The method accordingto claim 29, wherein said variable capacitor is composed of a pluralityof parallel-capacitances.
 31. A programmable/tunable active RC filter,comprising: a first controllable shunt device having an input terminal,a first output terminal and a second output terminal; a secondcontrollable shunt device having an input terminal, a first outputterminal and a second output terminal, wherein said second outputterminal of said second controllable shunt device is connected to saidsecond output terminal of said first controllable shunt device, saidfirst and second shunt controllable devices for determining a cut-offfrequency; a differential amplifier having two input terminals and twooutput terminals, wherein each of said two input terminals isrespectively connected to each of said first output terminals of saidfirst controllable shunt device and said second controllable shuntdevice; two first variable resistors R_(P), wherein one end of said twofirst variable resistors are respectively connected to a first inputsignal and a second input signal, and another end of said two firstvariable resistors are respectively connected to said input terminals ofsaid first controllable shunt device and said second controllable shuntdevice; two second variable resistors R_(f), wherein one end of said twosecond variable resistors are respectively connected to said two outputterminals of said differential amplifier, and another end of said twosecond variable resistors are respectively connected to said inputterminals of said first controllable shunt device and said secondcontrollable shunt device; and two capacitors, wherein each of said twocapacitors is respectively connected to said two input terminals andsaid two output terminals of said differential amplifier for forming aRC filter and being outputted from two output terminals of said twocapacitors.
 32. The programmable/tunable active RC filter according toclaim 31, further comprising: two third variable resistors R_(S), oneend of said two third variable resistors are respectively connected tosaid first output terminals of said first controllable shunt device andsaid second controllable shunt device, and another end of said two thirdvariable resistors feedback to said input terminals of said firstcontrollable shunt device and said second controllable shunt device. 33.The programmable/tunable active RC filter according to claim 31, furthercomprising: two fourth variable resistors R_(C) between said two firstvariable resistors and said input terminals of said first controllableshunt device and said second controllable shunt device, wherein one endof said two fourth variable resistors is connected to another end ofsaid two first variable resistors, and another end of said two fourthvariable resistors is connected to said input terminals of said firstcontrollable shunt device and said second controllable shunt device.34-40. (canceled)
 41. The programmable/tunable active RC filteraccording to claim 31, wherein said first controllable shunt device andsaid second controllable shunt device determine said cut-off frequencyby means of a synchronous control method.
 42. The programmable/tunableactive RC filter according to claim 32, wherein said two third variableresistors are trimmed for adjusting said active RC filter to a frequencytuning range.
 43. The programmable/tunable active RC filter according toclaim 33, wherein said two fourth variable resistors are trimmed foradjusting said active RC filter to a frequency band.
 44. Theprogrammable/tunable active RC filter according to claim 31, whereinsaid two first variable resistors are adjusted by means of a synchronouscontrol method for adjusting said programmable/tunable active RC filterto a voltage gain.
 45. The programmable/tunable active RC filteraccording to claim 31, wherein said two first variable resistors aretrimmed for programming said programmable/tunable active RC filter to avoltage gain.
 46. The programmable/tunable active RC filter according toclaim 32, wherein said two third variable resistors are trimmed by meansof a trimming method for adjusting said programmable/tunable active RCfilter to a frequency tuning range.
 47. The programmable/tunable activeRC filter according to claim 33, wherein said two fourth variableresistors are trimmed by means of a trimming method for adjusting saidprogrammable/tunable active RC filter to a frequency band.
 48. A dualmodule portable wireless communication system having aprogrammable/tunable active RC filter, comprising: a first controllableshunt device having an input terminal, a first output terminal and asecond output terminal; a second controllable shunt device having aninput terminal, a first output terminal and a second output terminal,wherein said second output terminal of said second controllable shuntdevice is connected to said second output terminal of said firstcontrollable shunt device; a differential amplifier having two inputterminals and two output terminals, wherein each of said two inputterminals is respectively connected to each of said first outputterminals of said second controllable shunt device and said secondcontrollable shunt device; two first variable resistors R_(P), whereinone end of said two first variable resistors are respectively connectedto a first input signal and a second input signal, and another end ofsaid two first variable resistors are respectively connected to saidinput terminals of said first controllable shunt device and said secondcontrollable shunt device; two second variable resistors R_(f), whereinone end of said two second variable resistors are respectively connectedto said two output terminals of said differential amplifier, and anotherend of said two second variable resistors are respectively connected tosaid input terminals of said first controllable shunt device and saidsecond controllable shunt device; two capacitors, wherein each of saidtwo capacitors is respectively connected to said two input terminals andsaid two output terminals of said differential amplifier for formingsaid programmable/tunable RC filter and then being outputted from twooutput terminals of said two capacitors; and two fourth variableresistors R_(C) between said two first variable resistors and said inputterminals of said first controllable shunt device and said secondcontrollable shunt device, wherein one end of said two fourth variableresistors is connected to another end of said two first variableresistors, and another end of said two fourth variable resistors isconnected to said input terminals of said first controllable shuntdevice and said second controllable shunt device; and a radiotransceiver, wherein said radio transceiver receives the signalsoutputting from said programmable/tunable active RC filter in said dualmodule portable wireless communication system. 49-50. (canceled)
 51. Thesystem according to claim 48, wherein said two fourth variable resistorsare adjusted by means of a synchronous control method for adjusting saidprogrammable/tunable active RC filter to a frequency band within theband of GSM or CDMA system.
 52. The system according to claim 51,wherein said two third variable resistors are adjusted by means of asynchronous control method for adjusting said programmable/tunableactive RC filter to a frequency tuning range.
 53. The system accordingto claim 50, wherein said two fourth variable resistors are trimmed foradjusting said programmable/tunable active RC filter to a frequency bandwithin the band of GSM or CDMA system.
 54. The system according to claim53, wherein said two third variable resistors are trimmed for adjustingsaid programmable/tunable active RC filter to a frequency tuning range.55. The system according to claim 48, wherein said two first variableresistors are adjusted by means of a synchronous control method foradjusting said programmable/tunable active RC filter to a voltage gain.56. The system according to claim 48, wherein said two first variableresistors are trimmed for programming said programmable/tunable activeRC filter to a voltage gain.